History
The first report of the haploid plant was published by Blakeslee ''et al.'' (1922) in ''Production of doubled haploids
Doubled haploids can be produced ''Genetics of DH population
In DH method only two types of genotypes occur for a pair of alleles, A and a, with the frequency of ½ AA and ½ aa, while in diploid method three genotypes occur with the frequency of ¼ AA, ½ Aa, ¼ aa. Thus, if AA is desirable genotype, the probability of obtaining this genotype is higher in haploid method than in diploid method. If n loci are segregating, the probability of getting the desirable genotype is (1/2)n by the haploid method and (1/4)n by the diploid method. Hence the efficiency of the haploid method is high when the number of genes concerned is large. Studies were conducted comparing DH method and other conventional breeding methods and it was concluded that adoption of doubled haploidy does not lead to any bias of genotypes in populations, and random DHs were even found to be compatible to selected line produced by conventional pedigree method.Applications of DHs plant breeding
Mapping quantitative trait loci
Most of the economic traits are controlled by genes with small but cumulative effects. Although the potential of DH populations in quantitative genetics has been understood for some time, it was the advent of molecular marker maps that provided the impetus for their use in identifying loci controlling quantitative traits. As theBackcross breeding
In backcross conversion, genes are introgressed from a donorBulked segregant analysis (BSA)
In bulked segregant analysis, a population is screened for a trait of interest and the genotypes at the two extreme ends form two bulks. Then the two bulks are tested for the presence or absence of molecular markers. Since the bulks are supposed to contrast in the alleles that contribute positive and negative effects, any marker polymorphism between the two bulks indicates the linkage between the marker and trait of interest. BSA is dependent on accurate phenotyping and the DH population has particular advantage in that they are true breeding and can be tested repeatedly. DH populations are commonly used in bulked segregant analysis, which is a popular method in marker assisted breeding. This method has been applied mostly to rapeseed and barley.Genetic maps
Genetic maps are very important to understand the structure and organization of genomes from which evolution patterns and syntenic relationships between species can be deduced. Genetic maps also provide a framework for the mapping of genes of interest and estimating the magnitude of their effects and aid our understanding of genotype/phenotype associations. DH populations have become standard resources in genetic mapping for species in which DHs are readily available. Doubled haploid populations are ideal for genetic mapping. It is possible to produce a genetic map within two years of the initial cross regardless of the species. Map construction is relatively easy using a DH population derived from a hybrid of two homozygous parents as the expected segregation ratio is simple, ''i.e.'' 1:1. DH populations have now been used to produce genetic maps of barley, rapeseed, rice, wheat, and pepper. DH populations played a major role in facilitating the generation of the molecular marker maps in eight crop species.Genetic studies
Genetic ratios and mutation rates can be read directly from haploid populations. A small doubled haploid (DH) population was used to demonstrate that a dwarfing gene in barley is located chromosome 5H. In another study the segregation of a range of markers has been analyzed in barley.Genomics
Although QTL analysis has generated a vast amount of information on gene locations and the magnitude of effects on many traits, the identification of the genes involved has remained elusive. This is due to poor resolution of QTL analysis. The solution for this problem would be production of recombinant chromosome substitution line, or stepped aligned recombinant inbred lines. Here, backcrossing is carried out until a desired level of recombination has occurred and genetic markers are used to detect desired recombinant chromosome substitution lines in the target region, which can be fixed by doubled haploidy. In rice, molecular markers have been found to be linked with major genes and QTLs for resistance to rice blast, bacterial blight, and sheath blight in a map produced from DH population.Elite crossing
Traditional breeding methods are slow and take 10–15 years for cultivar development. Another disadvantage is inefficiency of selection in early generations because ofCultivar development
Uniformity is a general requirement of cultivated line in most species, which can be easily obtained through DH production. There are various ways in which DHs can be used in cultivar production. The DH lines themselves can be released as cultivars, they may be used as parents in hybrid cultivar production or more indirectly in the creation of breeders lines and in germplasm conservation. Barley has over 100 direct DH cultivars. According to published information there are currently around 300 DH derived cultivars in 12 species worldwide. The relevance of DHs to plant breeding has increased markedly in recent years owing to the development of protocols for 25 species. Doubled haploidy already plays an important role in hybrid cultivar production of vegetables, and the potential for ornamental production is being vigorously examined. DHs are also being developed in the medicinal herb '' Valeriana officinalis'' to select lines with high pharmacological activity. Another interesting development is that fertile homozygous DH lines can be produced in species that have self-incompatibility systems.Advantages of DHs
The ability to produce homozygous lines after a single round recombination saves a lot of time for the plant breeders. Studies conclude that random DH’s are comparable to the selected lines in pedigree inbreeding.Friedt ''et al.'', 1986; Winzeler ''et al.'', 1987. The other advantages include development of large number of homozygous lines, efficient genetic analysis and development of markers for useful traits in much less time. More specific benefits include the possibility of seed propagation as an alternative to vegetative multiplication in ornamentals, and in species such as trees in which long life cycles and inbreeding depression preclude traditional breeding methods, doubled haploidy provides new alternatives.Disadvantages of DHs
The main disadvantage with the DH population is that selection cannot be imposed on the population. But in conventional breeding selection can be practised for several generations: thereby desirable characters can be improved in the population. In haploids produced from anther culture, it is observed that some plants are aneuploids and some are mixed haploid-diploid types. Another disadvantage associated with the double haploidy is the cost involved in establishing tissue culture and growth facilities. The over-usage of doubled haploidy may reduce genetic variation in breeding germplasm. Hence one has to take several factors into consideration before deploying doubled haploidy in breeding programmes.Conclusions
Technological advances have now provided DH protocols for most plant genera. The number of species amenable to doubled haploidy has reached a staggering 250 in just a few decades. Response efficiency has also improved with gradual removal of species from recalcitrant category. Hence it will provide greater efficiency of plant breeding.Tutorials
References
* Ardiel, G.S., Grewal, T.S., Deberdt, P., Rossnagel, B.G., and Scoles, G.J. 2002. Inheritance of resistance to covered smut in barley and development of tightly linked SCAR marker. Theoretical and applied genetics 104:457-464. * Blakelsee, A.F., Belling, J., Farhnam, M.E., and Bergner, A.D.1922. A haploid mutant in the Jimson weed, Datura stramonium. Science 55:646-647. * Burk, L.G., Gerstel, D.U., and Wernsman, E.A. 1979. Maternal haploids of Nicotiana tabacum L. from seed. Science 206:585. * Chen, F.Q., D.Prehn, P.M. Hayes, D.Mulrooney, A. Corey, and H.Vivar. 1994. Mapping genes for resistance to barley stripe rust (Puccinia striiformis f. sp. hordei). Theoretical and Applied Genetics. 88:215-219. * Friedt, W., Breun, J., Zuchner, S., and Foroughi-Wehr, B. 1986. Comparative value of androgenetic doubled haploid and conventionally selected spring barley line. Plant Breeding 97:56-63. * Guha, S., and Maheswari, S. C. 1964. In vitro production of embryos from anthers of Datura. Nature 204:497. * Immonen, S., and H. Anttila. 1996. Success in rye anther culture. Vortr. Pflanzenzuchtg. 35:237-244. * Kasha, K. J., and Kao, K. N. 1970. High frequency haploid production in barley (Hordeum vulgare L.). Nature 225: 874-876. * Kearsey, M. J. 2002. QTL analysis: Problems and (possible) solutions. p. 45-58. In: M.S. Kang (ed.), Quantitative genetics, genomics and plant breeding. CABI Publ., CAB International. * Maluszynski, M.., Kasha K. J., Forster, B.P., and Szarejko, I. 2003. Doubled haploid production in crop plants: A manual. Kluwer Academic Publ., Dordrecht, Boston, London. * Paterson, A.H., Deverna, J.W., Lanin, B., and Tanksley, S. 1990. Fine mapping of quantitative trait loci using selected overlapping recombinant chromosomes in an interspecies cross of tomato. Genetics 124:735-741. * Schon, C., M. Sanchez,T. Blake, and P.M. Hayes. 1990. Segregation of Mendelian markers in doubled haploid and F2 progeny of barley cross. Hereditas 113:69-72. * Thomas, W. T. B., B. Gertson and B.P. Forster. 2003. Doubled haploids in breeding p. 337-350. in :M. Maluszynski, K.J. Kasha, B.P. Forster and I. Szarejko (eds)., Doubled haploid production in crop plants:A Manual. Kluwer Academic Publ., Dordrecht, Boston, London. * Thomas, W.T.B., Newton, A.C., Wilson, A., Booth, A., Macaulay, M., and Keith, R. 2000. Development of recombinant chromosome substitution lines: A barley resource. SCRI Annual Report 1999/2000, 99-100. * Thomas, W.T.B., Powell, W., and Wood, W. 1984. The chromosomal location of the dwarfing gene present in the spring barley variety Golden Promise. Heredity 53:177-183. * Wang, Z., G. Taramino, D.Yang, G. Liu, S.V. Tingey, G.H. Miao, and G.L. Wang. 2001. Rice ESTs with disease-resistance gene or defense-response gene-like sequences mapped to regions containing major resistance genes or QTLs. Molecular Genetics and Genomics. 265:303-310. * William, K.J., Taylor, S.P., Bogacki, P., Pallotta, M., Bariana, H.S., and Wallwork, H. 2002. Mapping of the root lesion nematode (Pratylenchus neglectus) resistance gene Rlnn 1 in wheat. Theoretical and applied genetics 104:874-879. * Winzeler, H., Schmid, J., and Fried, P.M. 1987. Field performance of androgenetic doubled haploid spring wheat line in comparison with line selected by the pedigree system. Plant breeding 99:41-48. * Yi, H.Y., Rufty, R.C., Wernsman, E.A., and Conkling, M.C. 1998. Mapping the root-knot nematode resistance gene (Rk) in tobacco with RAPD markers. Plant Disease 82:1319-1322. {{refend Plant breeding Genetics Plant genetics